Steemit, Inc. currently wants to remove the barrier to entry for new users and they see ads as a barrier to entry.
If steemit.com starts monetizing by ads, anybody could set up steemit2.com without ads and display the same content.

there is a third reason. ads just don't make that much money unless youre a huge website.

As an example, i think i read once that 2p2 made about 300 a month from their banner advertising, and theyre a huge (if specialized) forum with a top 5 search engine exposures.

I agree with you about the speculative nature of the project being troubling (and, imo ultimately unsustainable) but ad revenue is simply not a viable solution.